feat: add duplicate alert check service

[jerrymusaga]

Summary

Closes #511

• Adds DuplicateAlertCheckService — a pre-save/pre-dispatch gate that fingerprints incoming alert events and matches them against recently-seen alerts within configurable time windows
• Three configurable severity behaviors per rule: block (drop silently), escalate (allow but upgrade priority), review (queue for manual resolution)
• Ships 3 sensible default dedup rules (exact-match block at 10 min, cross-source review at 5 min, critical escalation at 30 min) with full CRUD to add/update/delete custom rules
• alert.service.ts calls duplicateAlertCheckService.check() before persistEvent and record() after, so every persisted alert is tracked for future dedup
• REST API under /api/v1/duplicate-alert-check: rule management (/dedup-rules) and review queue (/review-queue + /review-queue/:id/resolve)
• Unit tests covering: block within window, allow outside window, source matching, severity escalation, review queue enqueue/resolve

Test plan

• npm run test:unit passes for duplicateAlertCheck.service.unit.ts
• POST an alert that matches an existing event within 10 min → confirm it is blocked (not persisted)
• POST a higher-priority re-trigger within 30 min → confirm priority escalated on the stored event
• GET /api/v1/duplicate-alert-check/dedup-rules returns the 3 default rules
• POST /api/v1/duplicate-alert-check/dedup-rules with a custom rule → rule appears in list and influences future checks
• GET /api/v1/duplicate-alert-check/review-queue?status=pending lists queued near-duplicates
• POST /api/v1/duplicate-alert-check/review-queue/:id/resolve with { action: "approved", reviewedBy: "..." } resolves the entry

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